In recent years, in order to reduce cost for driving circuits of a display panel, techniques for directly forming a scan line driving circuit (i.e., gate drivers) on a display panel substrate using a thin film transistor process have been implemented. However, reliable techniques for directly forming a data line driving circuit (i.e., data drivers) on the display panel substrate have not been successfully developed, so manufacturing cost for a display device cannot be further reduced. The main issue resides in that digital-to-analog (DAC) converters and buffers (implemented using operational amplifiers) which are included in the data line driving circuit of a conventional display device are analog circuits. Using thin film transistors (TFTs), whose characteristics are not sufficiently uniform, to implement such analog circuits directly on the display panel substrate may result in high cost.